Paramagnetic Nanocrystals: Remarkable Lanthanide-Doped

Nov 29, 2012 - Centre for Catalysis Research and Innovation and Department of Chemistry, ... She obtained her Ph.D. from McMaster University (supervis...
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Paramagnetic Nanocrystals: Remarkable Lanthanide-Doped Nanoparticles with Varied Shape, Size, and Composition Rebecca J. Holmberg, Tomoko Aharen, and Muralee Murugesu* Centre for Catalysis Research and Innovation and Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5 ABSTRACT: Magnetic nanoparticles have been developed in recent years with applications in unique and crucial areas such as biomedicine, data storage, environmental remediation, catalysis, and so forth. NaYF4 nanoparticles were synthesized and isolated with lanthanide dopant percentages, confirmed by ICP-OES measurements, of Er, Yb, Tb, Gd, and Dy that were in agreement with the targeted ratios. SEM images showed a distinct variation in particle size and shape with dopant type and percentage. HRTEM and XRD studies confirmed the particles to be crystalline, possessing both α and β phases. Magnetic measurements determined that all of the nanoparticles were paramagnetic and did not exhibit a blocking temperature from 2 to 300 K. The multifunctional properties of these nanoparticles make them suitable for many applications, such as multimodal imaging probes, up-conversion fluorescent markers, as well as MRI contrast agents.

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method. Because the atomic radii, common oxidation state, and general properties of the lanthanides are so similar, they are interchangeable to an extent. Thus, lanthanides are ideal for doping as they can be exchanged with one another and will contribute various different electronic properties.13 Theoretically, incorporating lanthanides into a nanocrystalline lattice encourages tuning of the size and crystalline phase, as well as the magnetic, electronic, and emission properties of the material. Thus, through lanthanide doping, the properties of nanocrystals can be suited to their desired application. Lanthanide nanoparticles have previously been developed with down-conversion and up-conversion luminescent properties in order to apply them in many areas, including medical imaging techniques.9,10,19−23 Down-conversion refers to a normal fluorescence response where the absorption of a photon leads to emission of a lower-energy, longer-wavelength photon. This phenomenon is less desirable as the light produced is in a wavelength range that is readily absorbed by biological tissues.24 Up-conversion describes the opposite process to down-conversion, where two or more photons are absorbed, leading to emission of a higher-energy, shorterwavelength photon.20,25−27 Photon up-conversion requires the long excited-state lifetimes, and evenly spaced ladder arrangements of the energy levels of lanthanide ions.20 In particular, this couple (Er, Yb) possesses the highest up-conversion efficiency due to the efficient energy transfer caused by the energy separation between the 2F7/2 ground state of YbIII and its 2F5/2 excited state matching the transition energies between 4 I11/2 and 4I15/2/4F7/2 states of ErIII.20 Through combining magnetism and fluorescence, we are able to correlate the

n recent years, nanoparticles have been extensively synthesized and studied and have been reported to possess countless potential niches. 1−7 The extensive array of applications of nanoparticles is highly dependent on their electronic and magnetic properties, and thus, these properties are vital to examine. Many nanoparticles have been isolated with the purpose of being used in magnetic applications,8−12 without necessarily investigating their magnetic properties or how they can be tuned to improve the particles for their applications. Through better knowledge of the properties of lanthanide nanoparticles within the field of molecular magnetism, a unique and innovative approach to the development and understanding of nanomagnetic materials can be implemented and could lead to the commercial application of such molecules. Although transition-metal systems are dominant in the field of nanoparticles, lanthanide-based particles continue to gain popularity due to their unique chemical and physical properties. Indeed, in such materials, due to the shielding of 4f electrons, fascinating magnetic and luminescent properties have been observed.13 Molecular lanthanide complexes with DyIII and TbIII have been studied in depth due to their large intrinsic magnetic anisotropy.14 Such magnetic anisotropy combined with a large amount of unpaired electrons could lead to magnet-like behavior, slow magnetic relaxation and magnetic hysteresis. By harnessing these unique characteristics of lanthanides, strong rare earth transition-metal magnets were created and widely applied in modern technologies.15−18 However, very little research has been developed that concentrates on highly anisotropic Tb- or Dy-based nanoparticles with a specific focus on magnetism. Another alluring aspect of lanthanide chemistry is that lanthanide ions can be employed in complexes that are isostructural to one another. This means that once a complex has been made with one lanthanide ion, it can theoretically be made with a neighboring lanthanide using the same synthetic © 2012 American Chemical Society

Received: October 2, 2012 Accepted: November 29, 2012 Published: November 29, 2012 3721

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findings illuminate potential applications of these nanoparticles as multimodal imaging probes, as up-conversion fluorescent markers, as well as MRI contrast agents. All nanocrystalline materials were synthesized under identical conditions in order to examine the effect that lanthanide doping had on the size, shape, crystalline, and magnetic properties. All measurements were performed from a single synthetic batch of each type of nanoparticle in order to allow accurate correlation between studied properties. Thus, measurements were performed using ICP-OES in order to determine the percentage of each dopant within the particles. SEM and TEM experiments were performed in order to investigate their size and shape, as well as their crystalline d-spacing (HRTEM). XRD experiments were performed in order to determine which crystalline phases were present in the particle distribution. Finally, SQUID measurements examined the magnetic properties of the particles. An important factor in studying the structural characteristics of these particles is their ionic radii, which can be seen according to the coordination number in Table 1. We have

properties of lanthanide nanoparticles to potential applications in magnetic resonance imaging (MRI) contrasting, labeled cell and protein separation, fluorescent markers, drug, gene, and radionuclide delivery, and so forth.28,29 In particular, MRI contrast agents have become an important area of study in biomedicine as they improve the abilities of an essential diagnostic and research technique. Contrast agents have been isolated using a varied array of particles with either superparamagnetic or paramagnetic properties.28,30−34 However, superparamagnetic contrast agents have been found to experience some drawbacks, dark signal due to negative contrast and distorted background images due to magnetic susceptibility artifacts of T2 MRI.30,35,36 Thus, paramagnetic nanoparticles that also possess up-conversion properties would have the unique ability to provide multimodal imaging capabilities. Therefore, our goal has been to create lanthanide nanoparticles with finely tuned size, shape, and magnetic properties. With this in mind, we have developed a methodology based on the synthetic method employed by Wang et al.37 Sodium yttrium fluoride (NaYF4) compounds were chosen due to their well-established synthetic methods, doping-induced up-conversion luminescence,37,38 and their potential for very interesting magnetic properties. These compounds were also desirable as extensively studied oxide-based lanthanide-doped nanoparticles have not yet achieved tunability in size, shape, and crystallinity.34 Because compounds with Y have been fully characterized and Y is easily interchanged with similarly sized lanthanide ions,13,37 these particles will facilitate the understanding of the effect of doping on the size, shape, and crystallinity of nanoparticles. The research goal has been to synthesize Gd/Tb/Dy/Er/Yb-doped up-converting nanoparticles and study their material characteristics, such as size, shape, crystallinity, and composition, in relation to their magnetic properties. The strategy employed is similar to that of alloying in materials, where different elemental compositions are investigated in order to optimize the properties for a certain application. In this case, we are using dopant compositions to investigate the optimal parameters for these nanoparticles to be used in magnetic applications.

Table 1. Effective Ionic Radii According to the Coordination Number for YIII, ErIII, YbIII, DyIII, TbIII, and GdIII39,40 effective ionic radii (Å) according to coordination number (N) ion

N=6

N=7

N=8

N=9

N = 10

YIII DyIII TbIII GdIII ErIII YbIII

0.90 0.91 0.92 0.94 0.89 0.87

0.96 0.97 0.98 1.00 0.95 0.93

1.02 1.03 1.04 1.06 1.01 0.99

1.08 1.09 1.10 1.11 1.06 1.04

1.14 1.15 1.17 1.12 1.10

carefully chosen the GdIII/TbIII/DyIII/ErIII/YbIII ions for the doping of NaYF4 particles based on the common stable +3 oxidation state of the lanthanide ions as well as similarities in coordination environment/numbers and, more importantly, close ionic radii. ICP-OES. ICP-OES measurements were performed on all samples, and the results can be seen in Table 2. These results display the percentage composition of each isotope within a solution of each compound. Thus, we can see the amounts of dopant that were successfully integrated into the system. They show that the particles contain amounts of each element that are very close to the desired ratios. The particles that were doped with Er and Yb show that these lanthanides are mainly replacing the Y, which is to be expected as its ionic radius is closer to that of the lanthanide dopants than that of Na. With an additional dopant of 15% Dy, we observe that Y is being replaced within the lattice further by about 10%, and Yb is also replaced by about 8%; thus, the Dy percentage is slightly higher than expected at 16.53%. This was followed by a two-fold increase in Dy doping (desired 30%), yielding 34.61%. We observed a further 10% replacement of Y and a lower percentage of Na. This suggests that the Dy doping in higher concentrations may cause a small percentage of Na to be replaced by Dy in the lattice. Following Dy, particles were doped with the same amounts of Tb. The 15% Tb-doped particles (16.69%) had a percentage of Tb that was very close to that of Dy in the 15% doped particles (16.53%). However, with a two-fold increase in Tb doping, we observe a lower percentage of Tb than desired (24.47%). This is most likely due to Tb being farther than Dy from Y in ionic radius. The closer

The research goal has been to synthesize Gd/Tb/Dy/Er/Ybdoped up-converting nanoparticles and study their material characteristics, such as size, shape, crystallinity, and composition, in relation to their magnetic properties. We were able to successfully synthesize percentages of dopants in agreement with the desired ratios. We also found a strong relationship between dopant type and concentration and the particle size, shape, and crystallinity. These findings are novel as they show that we can tune the particles to a desired specification through doping with lanthanides. In this Perspective, we show that these particles are indeed paramagnetic and not superparamagnetic between 2 and 300 K; thus, this study sheds light on the effects of lanthanide doping in nanoparticles and the resulting magnetic properties. These 3722

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Table 2. ICP-OES Analyte Percentage in Solution percentages of analytes in solution (%) percentage distribution NaYF4 NaYF4/2% NaYF4/2% NaYF4/2% NaYF4/2% NaYF4/2% NaYF4/2% NaYF4/2%

Er, Er, Er, Er, Er, Er, Er,

18% 18% 18% 18% 18% 18% 18%

Yb Yb, Yb, Yb, Yb, Yb, Yb,

15% 30% 15% 30% 15% 30%

Dy Dy Tb Tb Gd Gd

formula

Na

Y

Er

Yb

NaYF4 NaY0.70Er0.04Yb0.26F4 NaY0.59Er0.04Yb0.15Dy0.22F4 NaY0.41Er0.03Yb0.14Dy0.42F4 NaY0.50Er0.04Yb0.22Tb0.23F4 NaY0.29Er0.05Yb0.31Tb0.35F4 NaY0.64Er0.04Yb0.19Gd0.12F4 NaY0.47Er0.04Yb0.22Gd0.26F4

21.70 20.85 23.72 16.54 28.15 29.35 23.10 19.62

77.63 54.58 44.12 34.27 36.05 20.42 49.55 37.89

3.09 2.88 2.42 3.17 3.27 3.11 3.02

19.91 11.27 11.53 15.71 21.33 14.97 17.97

ionic radius to Y makes Dy more likely to have a higher composition as it preferentially replaces Y. Both particles doped with Tb were composed of more Na and Yb than those previously mentioned. This is most likely because Dy was closer in ionic radius to Yb and thus replaced more of it within the lattice. Finally, particles were doped with Gd in the same compositions as before. These particles showed a lower composition of Gd than was shown with Dy and Tb. The lower percentage of Gd present is most likely due to the fact that it has the farthest ionic radius from that of Y; thus, it is less favorable for replacement into the lattice structure. We can also observe that the values of Er were consistent except in the case of the Dy-doped nanoparticles, where they were replaced more due to the similarity of Er in radius to Dy. Overall, Dy-doped particles were closest to the expected composition values, and thus, Dy is the ideal lanthanide dopant for replacement of Y. SEM/TEM. SEM images were initially taken of undoped NaYF4 particles, which were the basis for the remainder of the particles synthesized. The representative image can be seen in Figure 1a. These NaYF4 particles have a shape that appears to be a distorted hexagon, clearly depicting uneven side lengths. From these analyses, we were able to carry out particle size distribution measurements (graph inset within Figure 1a), which displayed a mean size of 106.4 nm and a standard deviation of 8.4 nm. There was a very small portion of particles that were smaller in size, which, due to their infrequency, were not included in size distribution measurements. These small particles were 10−30 nm in size and were clustered together in agglomerated groups. This indicates that during nucleation, smaller particles agglomerate forming the observed larger nanocrystals; thus, we observed a small portion of these smaller particles remaining. The particles were then investigated using HRTEM, which showed the crystalline nature of the isolated material (Figure 1b). The crystallinity was further investigated by taking a Fourier transform of the HRTEM (inset within Figure 1b). The Fourier transform image shows a highly defined diffraction pattern from which we were able to calculate the d-spacing values. The average d-spacing values calculated for NaYF4 nanoparticles were 0.19, 0.30, and 0.51 nm. These values correspond to (101) and (100) hexagonal crystal orientations. These particles are quite different from those previously synthesized41,42 as they have a unique shape, as well as a wide size distribution, most likely due to the combined composition of cubic and hexagonal crystalline phases. The Fourier transform image also displays a large amount of hexagonal character and thus may indicate a preference toward this orientation. The NaYF4 synthetic strategy was then employed with different percentages of lanthanide dopants. Doping was

Dy

Tb

Gd

16.53 34.61 16.69 24.47 9.22 21.12

Figure 1. (a) SEM image of NaYF4 nanoparticles, illustrating the size of the particles (roughly 100 nm) and their rounded hexagonal shape. The inset is the particle size distribution graph. (b) HRTEM image with the inset Fourier transform.

performed with 2% Er and 18% Yb as this percentage composition was previously optimized for efficient upconversion luminescence.43 SEM images were then taken of these NaYF4 nanoparticles doped with 2% Er and 18% Yb. The representative image can be seen in Figure 2a. These particles are quite different from those previously synthesized11,37,44−46 as they are between spherical and hexagonal in shape and have a wide size distribution that is not often observed for this composition (20−30 nm range). These NaYF4/2% Er, 18% Yb particles have a shape that appears to be a combination of distorted hexagons (similar to NaYF4) and spherical particles. It is immediately apparent that there is a size change, from about 100 to 20 nm. This dramatic 3723

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Figure 2. (a) SEM image of NaYF4 (2% Er, 18% Yb) nanoparticles, illustrating their size (roughly 20 nm) and their rounded shape. The inset is the particle size distribution graph. (b) HRTEM image with the inset Fourier transform.

Figure 3. (a) SEM image of NaYF4 (2% Er, 18% Yb) doped with 15% Dy nanoparticles, illustrating the size of the particles (about 7 nm) and their rounded shape. The inset is the particle size distribution graph. (b) HRTEM image with the inset Fourier transform.

shift in size with the addition of lanthanide dopant can be attributed to the replacement of Y with smaller ions of Er and Yb within the lattice. The particles exhibit a mean size of 23.4 nm with a standard deviation of 3.1 nm, shown in the particle size distribution graph, which is inset within Figure 2a. We were then able to determine that the crystalline nature of these particles is retained with doping through HRTEM studies (Figure 2b). The average d-spacing values calculated for NaYF4/2% Er, 18% Yb nanoparticles were 0.20 and 0.33 nm. These values correspond to the (111*) cubic crystal orientation. The d-spacing values were obtained from the Fourier transform shown in the inset within Figure 2b. This diffraction pattern was far less defined than that of the NaYF4 samples. This is most likely due to less crystalline stacking, as well as an increase in amorphous material within the smaller particles. To these doped particles, an additional dopant of Dy at 15% composition was added and investigated in order to elucidate structural and property changes. This percentage was chosen as it provides a sufficient amount of dopant to potentially replace many different species within the lattice structure. These particles were studied using SEM (Figure 3a), which displayed a highly ordered assembly of very small nanoparticles. These NaYF4/2% Er, 18% Yb particles doped with 15% Dy have a shape that appears to be similar to that of the particles without 15% Dy dopant, rounded hexagons with uneven side

lengths, and some sphere-like particles. They exhibit a mean size of 7.1 nm with a standard deviation of 0.9 nm, which is shown by the particle size distribution graph inset within Figure 3a. Similar to previous findings, an additional type of dopant has reduced the particle size. These particles also had a very uniform size distribution. In order to elucidate whether Dy doping affects the particle crystallinity, HRTEM studies were performed (Figure 3b). A Fourier transform was taken of the HRTEM image and is shown in the inset in Figure 3b. From this diffraction pattern, we can calculate average d-spacing values, which were 0.19 and 0.30 nm, respectively. These values correspond to the (101) hexagonal crystal orientation and are similar to the d-spacing calculated for the parent particles (NaYF4), which have more values due to the crystalline stacking of thicker particles. In order to further investigate the influence of Dy on size and shape variation, a two-fold increase in the percentage of dopant was carried out. Remarkably, the synthesized particles have a near-perfect hexagonal shape, as depicted by the SEM images shown in Figure 4a,b. It is also apparent that these samples are significantly larger than previously doped samples. They exhibit a mean size of 120.0 nm with a standard deviation of 8.9 nm, which is shown by the particle size distribution graph inlaid within Figure 4a. Thus, the increased percentage of Dy proves to be ideal for the formation of well-defined large nanocrystals of these doped 3724

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orientation; however, the Fourier transform inset within Figure 4c displays apparent hexagonal character. From the Fourier transform, we were able to calculate an average d-spacing of 0.27 nm. These particles only have one d-spacing value, corresponding to cubic crystalline orientation, which is (200*). Samples were then doped with Tb because it neighbors Dy, with a slightly larger atomic radius. Our aim was to investigate how this would subsequently affect packing and thus overall crystallinity. SEM images were taken of NaYF4 nanoparticles doped with 2% Er, 18% Yb, and 15% Tb. The representative image can be seen in Figure 5a.

Figure 5. (a) SEM image of NaYF4 (2% Er, 18% Yb) doped with 15% Tb nanoparticles, illustrating the size (about 8 nm) and rounded shape of the particles. The inset is the particle size distribution graph. (b) HRTEM image with the inset Fourier transform.

Figure 4. (a,b) SEM images of NaYF4 (2% Er, 18% Yb) doped with 30% Dy nanoparticles, illustrating the size (about 120 nm) and hexagonal crystal form of the particles. The inset in (a) is the particle size distribution graph. (c) HRTEM image with the inset Fourier transform.

As expected, the neighboring Tb ions exhibit a similar shape and size distribution to the Dy-doped particles. Because the percent compositions found from ICP-OES experiments were similar between 15% doped Tb and Dy, we expect their structural properties to be similar. They exhibit a mean size of 7.8 nm with a standard deviation of 1.4 nm, which is shown by the particle size distribution graph inset in Figure 5a. This size is very close to that of 15% doped Dy nanoparticles (7.1 nm) yet slightly larger as Tb has a larger ionic radius than Dy (Table 1). In order to investigate the crystalline properties and compare them with those of other doped samples, HRTEM was performed on these particles (Figure 5b). The Fourier transform inset within Figure 5b is also similar to that of the particles doped with 15% Dy as it is not well-defined. The

materials. We observe, as in the case of NaYF4, smaller particles among the larger nanocrystals. In this case, there are many of these small particles; thus, a separate size distribution graph was obtained (Figure 4b). The mean size of these smaller particles was found to be 22.2 nm, with a standard deviation of 3.9 nm. Similar to the previous sample doped with Dy, these particles retain their crystallinity, as demonstrated by HRTEM (Figure 4c), but with a more refined diffraction pattern. The thickness of the samples made it challenging to show their crystalline 3725

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average d-spacing calculated for NaYF4/2% Er, 18% Yb nanoparticles doped with 15% Tb was 0.32 nm, which represents a (111*) cubic crystal orientation. To verify the previously observed trend with Dy in isolating large nanocrystals, we increased the dopant of Tb by two-fold to 30%. This resulted in confirmation of the highly hexagonal shaped nanoparticle being induced by 30% composition of dopant added. The size and shape of the particles were investigated using SEM (Figure 6a).

Finally, samples were doped with Gd in order to elucidate further the relationship between neighboring lanthanide dopants. The particles doped with 15% Gd were investigated using SEM (Figure 7a).

Figure 7. (a) SEM image of NaYF4 (2% Er, 18% Yb) doped with 15% Gd nanoparticles, illustrating the size (about 24 nm) and rounded shape of the particles. The inset is the particle size distribution graph. (b) HRTEM image with the inset Fourier transform.

Immediately, it becomes apparent that the Gd-doped particles have a different size and shape from those doped with Dy and Tb. These particles are more difficult to synthesize and do not possess the same hexagonal shape or size distribution. They are oval in shape and exhibit a mean size of 24.4 nm with a standard deviation of 2.5 nm, which is shown by the particle size distribution graph inset within Figure 7a. HRTEM was performed (Figure 7b), showing that the particles are indeed crystalline. The average d-spacing values calculated from the Fourier transform (inset in Figure 7b) are 0.17 and 0.31 nm, which represent the (111*) cubic crystal orientation. Finally, the Gd concentration was increased two-fold in order to compare Gd-doped particles with previously discussed Dyand Tb-doped particles. The representative SEM image shows that once again, the Gd-doped particles vary in size, shape, and distribution from those doped with Dy and Tb (Figure 8a). Unlike other 30% doped particles, those doped with Gd display a more rounded and less defined hexagonal shape. These particles are more similar in shape to the original NaYF4 than to Dy- and Tb-doped particles. They exhibit a larger mean size of 131.8 nm with a standard deviation of 5.1 nm, which is

Figure 6. (a) SEM image of NaYF4 (2% Er, 18% Yb) doped with 30% Tb nanoparticles, illustrating the size (about 120 nm) and the hexagonal shape of the particles. The inset is the particle size distribution graph. (b) HRTEM image with the inset Fourier transform.

Once again, we observe a strong trend in size and shape distributions between Tb- and Dy-doped nanoparticles. Thus, with controlled doping, we can tune the size and shape properties. The particles doped with 30% have an almost perfect hexagonal shape and are much larger than those doped with 15%. They exhibit a mean size of 122.9 nm with a standard deviation of 9.6 nm, which is shown by the particle size distribution graph inset in Figure 6a. They retain their crystallinity, as shown by HRTEM (Figure 6b). The average d-spacing values calculated from the inset Fourier transform were 0.20 and 0.33 nm, which correspond to the (111*) cubic crystal orientation. These two d-spacing values, as opposed to 30% Dy, which only had one value, are most likely attributed to a much wider size distribution found for Tb-doped nanoparticles. 3726

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two types of crystal phases can be observed for NaYF4, which are shown in Figure 9.

Figure 9. Cubic crystal form of NaYF4 (a) and hexagonal crystal form of NaYF4 (b). Adapted by permission from Macmillan Publishers Ltd.: Nature, copyright (2010), http://www.nature.com/nature/journal/ v463/n7284/full/nature08777.html.37. Figure 8. (a) SEM image of NaYF4 (2% Er, 18% Yb) doped with 30% Gd nanoparticles, illustrating the size (about 130 nm) and rounded hexagonal crystal form of the particles. The inset is the particle size distribution graph. (b) HRTEM image of NaYF4 (2% Er, 18% Yb) doped with 30% Gd nanoparticles, with the inset Fourier transform image.

The cubic crystal phase is often referred to as α, whereas the hexagonal crystal phase is referred to as β. It is most likely that Y will be replaced by dopant ions due to the isostructurality of lanthanides, the close atomic and ionic radii values, and their +3 oxidation state. However, because the dopant lanthanides with a coordination number of 9 have effective ionic radii in the range of 1.04−1.11 Å (Table 1),39,40 and YIII and NaI have atomic radii of 1.08 and 1.02 Å, respectively,40 theoretically, Na could also be replaced by lanthanide dopants; however, due to the +3 oxidation state of the dopant ions, Y replacement would be favored.37 If substitution occurs of YIII with larger ions in the lattice, we would expect a systematic peak shifting effect to lower angles within the XRD pattern as compared to the standard due to unit cell expansion.40,49 Results from powder X-ray diffraction experiments displayed multiple crystalline phases within the particles, cubic (α) and hexagonal (β) phases. In order to assign the peaks corresponding to particular phases, PDXL software equipped with the RIGAKU apparatus was used with the ICDD database. The peaks that have been marked with an asterisk represent lattice planes that are cubic (α) phase (JCPDS 006-0342), and all other peaks represent lattice planes that are hexagonal (β) phase (JCPDS 016-0334). We can also observe the prevalence of one cubic phase peak in each pattern, representing the (200*) lattice plane. The XRD pattern for the parent particles (NaYF4) can be seen in Figure 10. This pattern shows a strong peak representing the hexagonal (β) phase (210), as well as two smaller peaks representative of the β phase (110) and (101), which were also seen in the d-spacing values from HRTEM

shown in the inset in Figure 8a, the particle size distribution graph. Once again, this shows that larger ionic radii will increase the lattice and, therefore, the particle size. In order to compare their crystallinity, HRTEM studies were performed (Figure 8b). The average d-spacing value, calculated from the Fourier transform inset in Figure 8b, was 0.32 nm, which represents the (111*) cubic crystal orientation. From electron microscopy and diffraction experiments, we were able to observe that Dy is the best dopant for site replacement. Although Gd- and Tb-doped particles are close to the desired percentages of the dopant (Table 2), Dy remains the most accurate to the desired percentages; this is consistent with the superior size, shape, and crystallinity data observed for Dy-doped particles by electron microscopy and diffraction experiments. This could be related to the closeness in size of DyIII ions to YIII ions, thus facilitating the doping of the particles. The variations in size will be further explained through a correlation between the size and crystalline phases in the following discussion on powder X-ray diffraction experiments. XRD. In order to confirm the crystallinity and phases observed by HRTEM, powder X-ray diffraction (XRD) measurements were employed. From previous studies,37,47,48 3727

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Figure 10. Powder X-ray diffraction pattern for NaYF4 nanoparticles. Selected 2Θ regions from 25 to 35 and 45 to 50°.

Figure 12. Powder X-ray diffraction pattern for NaYF4/2% Er, 18% Yb, 15% Dy nanoparticles. Selected 2Θ regions from 25 to 35 and 44 to 50°.

studies. There is also a high-intensity peak representing the cubic (α) phase (200*) and a low-intensity cubic phase peak (111*). Thus, these particles show a majority of their peak intensity corresponding to the cubic crystal phase, with significant intensity corresponding to the hexagonal phase. Conversely, the pattern for NaYF4/2% Er, 18% Yb particles (Figure 11) was different from those previously observed. This

Figure 13. Powder X-ray diffraction pattern for NaYF4/2% Er, 18% Yb, 30% Dy nanoparticles. Selected 2Θ regions from 25 to 35 and 45 to 50°.

The pattern for the particles doped with 30% Dy (Figure 13) shows a strong peak representing the cubic (α) phase (200*), as well as four low-intensity peaks representative of the β phase, (110), (101), (201), and (210). This pattern is similar to that of the parent particles (NaYF4); however, the intensity of the (200*) peak is larger, causing the other peaks to be comparatively smaller, and there is a (201) peak additionally present representing the β phase. Comparison between XRD and electron microscopy shows agreement between the defined hexagonal structure of 30% doped Dy particles and the pattern transformation from the α to β phase. Thus, this preference toward the hexagonal phase as nucleation progresses could explain the larger sizes and hexagonal shapes of the undoped sample and that doped with 30% Dy. The pattern for NaYF4/2% Er, 18% Yb, 15% Tb particles can be seen in Figure 14. This pattern shows a strong peak representing the cubic (α) phase (200*), as well as two other broad peaks representing the α phase, (111*) and (220*). There is also a small peak that represents the β phase (210). This pattern is similar to that of 15% Dy particles. Both patterns show a prominent (200*) cubic peak, with noisy and wide cubic regions, as well as one small hexagonal (210) peak. The cubic phase was also favored in the d-spacing value that

Figure 11. Powder X-ray diffraction pattern for NaYF4/2% Er, 18% Yb nanoparticles.

pattern shows a single strong peak representing the cubic (α) phase (200*), with no other peaks being visible. This is mainly due to the intensity of the (200*) peak as it would dwarf any subsequent peaks, as well as preferential crystal orientation. Thus, these particles are mainly composed of the cubic crystal phase. This was expected as the d-spacing values for these particles corresponded to the cubic phase. The pattern for NaYF4/2% Er, 18% Yb, 15% Dy particles (Figure 12) also shows a strong peak representing the cubic (α) phase (200*), with a wide region of intensity representing (111*). One other small peak is visible, which represents the hexagonal cubic phase (210). Thus, these particles are mainly composed of the cubic crystal phase with a smaller amount of hexagonal phase. This is expected because the d-spacing values were similar to those for the undoped parent particles, and thus, we find that both types of particles contain (210) and (200*) crystal orientations. 3728

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Figure 14. Powder X-ray diffraction pattern for NaYF4/2% Er, 18% Yb, 15% Tb nanoparticles. Selected 2Θ regions from 25 to 35 and 45 to 50°.

Figure 16. Powder X-ray diffraction pattern for NaYF4/2% Er, 18% Yb, 15% Gd nanoparticles. Selected 2Θ regions from 25 to 35 and 45 to 50°.

represented the (111*) crystal orientation. The diffraction pattern from HRTEM was not well-defined; thus, these particles may have very few crystalline phases other than (200*). As expected, the pattern for NaYF4/2% Er, 18% Yb, 30% Tb particles (Figure 15) is somewhat similar to that of the particles

(111*). Thus, the peak intensities of these particles show a preference to the hexagonal crystal phase. This is consistent with previous findings for increasing Gd dopant concentration.37 Finally, a pattern was also taken for NaYF4/2% Er, 18% Yb, 30% Gd particles; however, it showed extremely low-intensity, broad peaks. This can most likely be attributed to slow nucleation and growth, as well as a more amorphous and less crystalline particle than was found for other dopant concentrations. It was, however, identifiable that there are most likely two types of crystal phases (α and β) present due to the observation of very broad, low-intensity peaks. Thus, we can conclude that the majority of the particles have both crystal phases (α and β) present in their structure. While indexing the spectra, there was a noticeable shift to lower angles for most of the peaks, thus showing that Y is being replaced by larger ions within the lattice.37 All of these particles had similar XRD patterns and d-spacing values, with the parent compound having an additional d-spacing value, most likely due to crystalline stacking as well as a partially preferred orientation. It also became apparent from XRD studies that the two types of particles that were nearly perfect hexagons in shape (NaYF4/ 2% Er, 18% Yb, doped with 30% Dy and 30% Tb), were potentially shifting from the cubic to hexagonal phase with nucleation and growth. This could also explain the size similarities between these particles and those that were becoming more hexagonal in shape (NaYF4 and NaYF4/2% Er, 18% Yb, doped with 30% Gd). Both spectra for Dy-doped compounds were consistent with what was expected and showed clear, well-defined peaks, again confirming that Dy is the optimal dopant. Magnetic Studies. All doped samples were characterized using ZFC/FC measurements (Figure 17) and inverse susceptibility measurements (Figure 18) in order to discern their magnetic properties (superparamagnetic versus paramagnetic). Lowtemperature hysteresis measurements (Figures 19 and 20) were also performed in order to elucidate whether the particles would exhibit coercivity and thus be superparamagnetic or whether they were paramagnetic. Because superparamagnetism occurs below a blocking temperature, low-temperature experiments must be performed in order to confirm whether the magnetic behavior is following a field (paramagnetic) or

Figure 15. Powder X-ray diffraction pattern for NaYF4/2% Er, 18% Yb, 30% Tb nanoparticles.

doped with 30% Dy. This pattern shows a strong peak representing the cubic (α) phase (200*) and a very small α peak (111*), as well as a tiny β peak (210). As these particles were very similar in size, shape, and crystallinity to the 30% doped Dy particles, it is surprising that they do not possess more hexagonal crystalline properties. This may be attributed to the nucleation being in an earlier stage; thus, the hexagonal phase preference has not yet been well-developed. The two dspacing values were similar to those for 15% Tb. Doping with Gd caused a change in the characteristics of the particles from those of the parent particles or particles doped with other lanthanides. The pattern for the particles doped with 15% Gd (Figure 16) shows very wide and broad peaks, which indicates that these particles were not well-defined in crystallinity. This pattern shows three peaks representing the hexagonal (β) phase, (110), (101), and (210). There are also two small peaks representing the cubic (α) phase, (210*) and 3729

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Figure 17. Zero-field cool/field cool (ZFC/FC) experiment performed on the NaYF4/2% Er, 18% Yb parent compound, as well as those doped with 15 and 30% Dy, 15 and 30% Tb, and 15 and 30% Gd. The value of the applied field was 100 Oe.

Figure 19. Hysteresis experiments performed at 2 K on the NaYF4/2% Er, 18% Yb parent compound, as well as those doped with15 and 30% Dy, 15 and 30% Tb, and 15 and 30% Gd.

Figure 20. Zoomed-in view of hysteresis experiments performed at 2 K on the NaYF4/2% Er, 18% Yb parent compound, as well as those doped with15 and 30% Dy, 15 and 30% Tb, and 15 and 30% Gd.

Figure 18. Inverse susceptibility measurements versus temperature performed on the NaYF4/2% Er, 18% Yb parent compound, as well as those doped with 15 and 30% Dy, 15 and 30% Tb, and 15 and 30% Gd. The value of the applied field was 100 Oe.

paramagnetic nature of these compounds makes them suitable for many applications; thus, the magnetic properties of lanthanide-doped nanoparticles are essential in exploring and improving them for particular applications. In conclusion, the presented synthetic strategy was successful; we demonstrated that highly anisotropic Dy and Tb ions can replace Y to produce highly crystalline particles with shape and size control. Initially, through the employment of ICP-OES measurements, we were able to determine that the desired nanoparticles had been successfully synthesized and isolated with dopant percentages in agreement with the targeted ratios. Through this knowledge, we were able to correlate the amount of Y replaced with the efficiency of the lanthanide dopant. The Dy-doped particles were found to have the closest composition to that which was desired. Thus, Dy is an efficient dopant for replacement of Y, which is logical as it has the closest ionic radius to Y. The particles were then studied using SEM, TEM, and HRTEM in order to elucidate and compare their shape, size,

whether it is retained after the removal of a field (superparamagnetic). Due to the diamagnetic nature of Y, magnetic measurements were not performed on NaYF4 particles. Thus, the experiments were performed initially on NaYF4/2% Er, 18% Yb particles, followed by all further doped particles (Figures 17−20). The ZFC/FC plots superimpose atop one another, indicating that there is no superparamagnetic behavior and no blocking temperature between 2 and 300 K. Inverse susceptibility plots show linear relationships with temperature, with the exception of slight nonlinearity below 100 K for the sample doped with 30% Dy, illustrating possible dipole−dipole Dy interactions. Similar behavior has previously been observed in molecular magnets.50 Magnetization versus field (hysteresis) experiments did not show any loop-opening phenomena (Figures 19 and 20), or coercivity, at low temperature (2 K); thus, the compounds display paramagnetic behavior and not superparamagnetic behavior at or above 2 K. The confirmed 3730

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methanol, followed by redispersal in cyclohexane. The particle suspensions were slightly opaque and ranged from off-white to a light beige color. ICP-OES. Inductively coupled plasma optical emission spectrometry measurements were performed using a Varian Vista Pro CCD-ICP-OES spectrometer. These measurements allowed for the determination of the percent composition of lanthanide dopants within the particles. Each sample was digested separately, in duplicates, prior to analysis. The duplicates agreed well and were averaged to obtain the final analyte relationship calculations. Each element was monitored at more than one wavelength in order to validate the data and rule out interferences. All concentrations were calculated in ppm (w/w basis) and are presented as percentage composition values. SEM/TEM. Scanning electron microscopy images were taken to elicit the size and shape of the particles using a JSM-7500F FESEM (JEOL), and transmission electron images were taken to obtain better resolution images of particles with a JEM2100F FETEM (JEOL) housed at CCRI-University of Ottawa. HR-TEM (JEOL JEM-2100F) was used to study the intrinsic crystallography of the samples. XRD. X-ray powder diffraction experiments were performed to study the crystalline phases of the particles and employed a RIGAKU Ultima IV, housed at the University of Ottawa, equipped with a Cu Kα radiation source (λ = 1.541836 Å) and a graphite monochromator. Scanning of the 2Θ range was performed from 15 to 48°, depending on the particular sample. SQUID. Magnetic measurements were performed using a Quantum Design SQUID magnetometer MPMS-XL7, operating between 2 and 300 K for dc-applied fields ranging from −7 to 7 T. Zero-field cool/field cool and hysteresis measurements were performed on centrifuged and dried nanoparticle samples, NaYF4/2% Er, 18% Yb (14.2 mg), NaYF4/2% Er, 18% Yb, 15% Dy (17.6 mg), NaYF4/2% Er, 18% Yb, 30% Dy (9.5 mg), NaYF4/2% Er, 18% Yb, 15% Tb (7.3 mg), NaYF4/2% Er, 18% Yb, 30% Tb (14.0 mg), NaYF4/2% Er, 18% Yb, 15% Gd (9.9 mg), and finally NaYF4/2% Er, 18% Yb, 30% Gd (6.2 mg). Samples were all wrapped within a polyethylene membrane. The zero-field cool/field cool measurements were taken between 2 and 300 K, with an applied field of 100 Oe, and the hysteresis measurements were performed at 2 K.

and crystallinity. We observed from the SEM and TEM images as well as the particle size distribution graphs that the size of the nanoparticles became larger with equivalent percentages of larger radii lanthanide dopants. Thus, the size of the lanthanide dopant will selectively control the size of the nanocrystalline material. It also became apparent that with larger dopant percentages, the particles grew in size and became more defined in shape (hexagons). All of these particles had similar XRD patterns and d-spacing values, with the parent compound having an additional d-spacing value, most likely due to crystalline stacking as well as partially preferred orientation. The particles were determined to be crystalline, with the majority of particles experiencing both cubic and hexagonal phases in their structures. The observed size similarities between the particles that were hexagonal or becoming more hexagonal in shape (NaYF4 and NaYF4/2% Er, 18% Yb, doped with 30% Dy, 30% Tb, and 30% Gd) can also be correlated with a shift toward the hexagonal crystal phase. Both spectra for Dy-doped compounds were consistent with what was expected and showed clear, well-defined peaks, again confirming that Dy is the optimal dopant. This is also important regarding upconversion fluorescent and magnetic properties of the nanocrystals as they can vary depending on crystalline phase. The magnetic measurements showed that there was no superparamagnetic behavior and no blocking temperature between 2 and 300 K. Magnetization versus field (hysteresis) experiments showed no loop-opening phenomena, or coercivity, at low temperature (2 K); thus, the compounds displayed paramagnetic behavior and not superparamagnetic behavior at or above 2 K. Thus, the multifunctional properties of these lanthanide-doped particles make them promising materials in applications, such as MRI contrast agents, optoelectronic devices, and light display applications.

The multifunctional properties of these lanthanide-doped particles make them promising materials in applications, such as MRI contrast agents, optoelectronic devices, and light display applications.

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AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected].

EXPERIMENTAL SECTION All reagents were purchased from the following sources: Alfa Aesar, Acros Organics, Strem Chemicals, and Sigma Aldrich. All reagents were employed without further purification. Synthesis. The synthetic method employed was derived from previous works,35,36 with variations in the concentrations of the dopant and in lanthanide ions used as dopants (Dy, Tb, and Gd). A molar percentage of rare earth chloride hexahydrate, according to the desired amount of dopant, in 2 mL of methanol, was added to 3 mL of oleic acid and 7 mL of 1octadecene. The mixture was heated to 160 °C for 30 min, cooled to room temperature, and stirred with ammonium fluoride and sodium hydroxide in 5 mL of methanol for 30 min. The mixture was then put under vacuum to remove methanol and heated under N2 to 300 °C for 1.5 h. Once cooled, nanoparticles were precipitated by adding ethanol. They were then centrifuged from the mixture and washed with ethanol/

Funding

We gratefully acknowledge financial support toward this project from the NSERC of Canada. Notes

The authors declare no competing financial interest. Biographies Rebecca Holmberg is a doctoral student in the research group of Dr. Murugesu at the University of Ottawa. She completed her M.Sc. and B.Sc. (Hons.) at Queen’s University and is now working on magnetic nanomaterials with applications in biomedicine and information storage. Tomoko Aharen is a Vision 2020 Postdoctoral Fellow in the research group of Dr. Murugesu at the University of Ottawa. She obtained her Ph.D. from McMaster University (supervisor, Prof. John E. Greedan) and is now working on the magnetism and medical applications of metal organic frameworks (MOFs). 3731

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Muralee Murugesu is an Associate Professor of Inorganic Chemistry at the University of Ottawa. He works in the field of molecular magnetism, with a focus on the design and development of new synthetic methods towards high-energy barrier single-molecule magnets and nanomaterials. Website: http://mysite.science.uottawa. ca/mmuruges/

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